The influence of dopants on loss tangent of polycrystalline alumina ceramics

Polycrystalline aluminas with various concentrations of oxide dopants CaO, MgO, and TiO2, ranging from 0.05 wt.% to 5 wt.%, as well as pure alumina references were prepared by slip casting from suspensions with various solid loading (30, 35, and 40 vol.% of Al2O3), and sintered at 1350 °C for 1 h as low loss tangent electroceramics. As the key parameter for intended application the loss tangents were measured at room temperature in the frequency range between 1 and 200 kHz. In the case of pure alumina the lowest value of loss tangent was achieved in the materials with minimum residual porosity. The values of loss tangents of doped materials were influenced by the concentration of dopants. The addition of 0.05 and 0.5 wt.% of MgO and CaO decreased the value of loss tangent in the whole frequency range. The effect was related to more homogenous microstructure with lower concentration of defects (pores) resulting from the addition of dopants. The increased values of loss tangent in the materials with higher level of additives (5 wt.%) are related to lower density of the materials, and the presence of residual porosity. Other contributing factors are the formation of secondary phases (calcium, magnesium and titanium aluminates), and the increased concentration of lattice defects due to incorporation of atoms with different valencies to alumina crystal lattice.